Source said ISIL, on Sunday, detonated more than 30 tonnes of explosives at the temple, the largest and one of the most significant structures in the UNESCO-listed city.
The British-based Syrian Observatory for Human Rights and activists on the ground also said that the temple has been damaged in the blast.
The Temple of Bel stands just outside the main area of the city, whose column-lined principle street leads to its gate.
Its richly decorated central shrine area was regarded as well preserved.
The destruction comes a week after the group blew up Palmyra's 2,000-year old temple of Baal Shamin, causing much damage.
Satellite images captured several days ago have confirmed the destruction of the Roman-era Baal Shamin temple.
It was another important site in Palmyra, known as the "Pearl of the Desert", a previously well-preserved archaeological oasis 210km northeast of Damascus.
Earlier this month, ISIL beheaded Khaled Asaad, a respected 82-year-old archaeologist who worked for 50 years as head of antiquities in Palmyra.
ISIL captured Palmyra from government forces in May.
Source: Agencies
